Cross winds affect the paths of various things moving through air. For example, they influence a bullet fired at a distant object, a boat moving across water, or an airplane approaching a landing strip. The problem with bullets is first described: When a bullet is fired from a firearm, such as a common rifle, the bullet is subjected to forces which deflect it from a desired straight line path which runs along the centerline of the barrel of the gun to the target. That straight line path is called here the Line of Sight (LOS). Influences which cause a bullet to deviate from the LOS include gravity and wind; and gun sights often have built-in means for selectively compensating for such. There are other effects on the travel path of a bullet, e.g., those induced by the spin of a bullet, which are beyond the scope of the present discussion.
Suppose a bullet is fired horizontally. Gravity causes the bullet to drop vertically below the LOS. To compensate for gravity, the exit end of the barrel of the gun is elevated from the LOS so the bullet first angles upwardly, and travels along an arc path trajectory to the target. The amount of elevation adjustment depends on the distance to the target (the “range”) and the velocity of the bullet; in sum, it depends on the time of travel. Since manufactured cartridges impart consistent velocities to bullets, cartridge makers are able to provide tables showing the trajectory of bullets according to range. Thus, the effect of gravity is predictable and correctable, to the extent range is known.
A wind transverse to the LOS causes a bullet to move with the wind and the bullet impact point is laterally displaced from the aim point. To compensate for transverse wind (which as a simplification in this discussion is assumed to be blowing parallel to the earth surface), the barrel of a gun is angled from the LOS in the horizontal plane, so the bullet travels along an arc curve trajectory BP, as illustrated in FIG. 1. Angling of the barrel, by adjusting or “clicking” the gun sight, is called a windage adjustment. (In firearms parlance, windage refers both to the deflection caused by wind, and the adjustment to compensate for the deflection.) The amount of adjustment which is needed depends on (a) the velocity and direction of the wind; (b) the bullet velocity; and (c) the range. The range (c) divided by the average velocity (b) is the time of travel. Heretofore, small arms gun sights have been adjusted for windage based on the shooter's estimate of the wind velocity and the range, or based on the observed deviation of a test shot. The deviation can be significant. For example, the approximate deviation for a .30-06 caliber bullet moving in a crosswind of 5 mph (224 cm/s) is about 79 cm over a range of 1000 m.
Firing a test shot and observing the windage deflection is often not feasible. First, the point of impact of the test shot may not be observable. Second, the test shot may compromise the purpose of the shooter, such as by alerting hunting prey and causing the prey to bolt away before a second windage-corrected shot can be made.
In a present day situation, a shooter may have information about wind velocity at the shooter's location (as from a wind gage he or she holds) and the distance to the target. Skilled shooters will also observe natural signs, such as the bending of vegetation, motion of dirt or water, etc. However, there can be variation in wind velocity and direction along the path, and thus a windage adjustment based on velocity at the shooter location or a way point may be wrong. Frequently, a windage adjustment is based on judgment rather than exact measurement. Thus, there is a need for a better means of determining and correcting for the effects of wind along the flight path of a bullet. The present invention seeks to provide that.
Turning briefly to aircraft, winds obviously affect their flight paths. Pilots commonly estimate and correct for the integrated effects of wind along the path between the plane and a destination, also referred to in this application as the target point. The need for immediate information and corrective action is acute when a plane is landing. Cross winds can cause a plane to move sideways with respect to its alignment with the runway. Wind shear, or localized changes in wind direction, may have vertical and horizontal components. Flying of large airplanes and compensating for wind during landing is now sophisticated. For example, in the 1990's airborne wind shear detectors using X-band Doppler radar were commercialized. However, for smaller planes it could be useful to have an alternative tool for detecting the integrated wind effects along the flight path, from either the cockpit of the plane or from the ground.
Turning to sailboats and motor boats, they also are deflected from their intended paths by transverse winds. Expert sailors are often very skilled at “reading” the magnitude and direction of the wind. They may observe pennants, the water surface, birds, other boats, etc. Of course, when the just-traveled portion of the travel path is known, as from electromagnetic positioning systems or other navigation means, the just-past wind effect may be determined—provided it can be separated from any deflection induced by water current. Nonetheless, it would be beneficial to have an accurate measure of the integrated magnitude of wind along the intended path, to the extent present means do not provide that, or to the extent the present means are costly.
Implicit in resolving the foregoing situations is measuring the air velocity. Knowing the wind may be of interest for other reasons than guiding an article through airspace, e.g., for meteorological purposes, effect on heating and cooling of things, etc. Thus, there is general continuing need for measuring well the motion of air in the atmosphere, and a generalized and continuing need for measuring the motion of fluids transverse to an observation point, for instance, or to determine volume flow of fluids, both within and without conduits in commercial and industrial processes.